Emission control devices, such as an underbody catalyst, coupled to an exhaust passage of an internal combustion engine reduce combustion by-products such as oxides of nitrogen, carbon monoxide, and hydrocarbons. Engine cold-start emissions generated before light-off of the underbody catalyst may contribute to a significant percentage of the total exhaust emissions. Efficiency of the underbody catalyst may be affected by exhaust temperature, and may be sub-optimal outside a specific temperature range. Also, functionality of the catalyst may be adversely affected by a higher than threshold oxygen loading in the catalyst.
Accordingly, various approaches have been developed for selectively routing exhaust through an exhaust catalyst based on exhaust temperature. One example approach, shown by Servati et al. in U.S. Pat. No. 5,377,486 involves, during cold-start conditions, routing exhaust first through a smaller catalyst before routing the exhaust through the underbody (main) catalyst, and after attainment of light-off temperature of the main catalyst, bypassing the smaller catalyst, and routing exhaust directly through the main catalyst. The smaller catalyst may attain light-off earlier than the main catalyst and by flowing exhaust first through the smaller catalyst, emissions quality may be increased. By bypassing the smaller catalyst after attainment of light-off temperature of the main catalyst, any damage to the smaller catalyst caused by a higher than threshold exhaust temperature may be reduced.
However, the inventors herein have recognized potential issues with such a system. As one example, due to the configuration of the system, irrespective of the exhaust temperature, the main catalyst is not bypassed. As such, owing to a coating on the underbody catalyst surface, the catalyst may have higher conversion efficiencies in a defined exhaust temperature range. As a result, at exhaust temperatures that are lower than or higher than the defined range, the functionality of the underbody catalyst may be reduced. In addition, the flow of hot exhaust may cause damage to catalyst components. As another example, as exhaust flows through the catalyst during cold-start conditions, water from exhaust components upstream of the main catalyst may condense on the catalyst and extract energy from the catalyst for evaporation, which may further affect catalyst functionality, and delay the attainment of light-off temperature. Further, during engine operating conditions such as a deceleration fuel shut off (DFSO), the engine may be operated un-fueled with air still being pumped through the cylinders. As a result, a higher concentration of oxygen may reach the catalyst causing oxygen saturation of the underbody catalyst. The oxygen saturation may result in reduction in the catalyst's ability to convert NOx adsorbed on the catalyst, thereby affecting emissions quality.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. In one example, the issues described above may be addressed by a method for an engine, comprising: during engine non-fueling conditions, flowing exhaust through a bypass passage while bypassing an exhaust underbody catalyst positioned in a main exhaust main passage, via a valve positioned downstream of the catalyst; and during engine fueling conditions, selectively flowing exhaust through the bypass passage based on each of a temperature and water content of the exhaust. In this way, by opportunistically bypassing the underbody catalyst during conditions which may adversely affect functionality of the catalyst, emissions quality may be improved.
In one example, the exhaust system may include a front catalyst and a main underbody catalyst coupled to the main exhaust passage downstream of the exhaust manifold. A bypass passage may be coupled to the main exhaust passage, parallel to the underbody catalyst, the bypass including a diverter. After flowing through the front catalyst, exhaust may either flow through the underbody catalyst, or flow through the bypass passage, bypassing the underbody catalyst. Routing of exhaust through the main passage or the bypass passage may be regulated via adjustments to a position of the diverter valve. For example, during cold-start conditions, the diverter valve may be adjusted so that exhaust may first bypass the underbody catalyst until water present in the exhaust flow has evaporated, and then the diverter valve may be readjusted so that exhaust may be routed through the underbody catalyst. Heat from the exhaust may be used to heat the underbody catalyst and attain the light-off temperature. In comparison, exhaust may be routed through the underbody catalyst during higher cylinder air mass conditions, as well as higher exhaust temperature conditions. In one example, during cooler exhaust temperature conditions, the exhaust may be routed to bypass the underbody catalyst in order to maintain operating temperature of the underbody catalyst above a desired operating temperature, as well as to reduce water in the exhaust from lowering the catalyst temperature. Similarly, during hotter exhaust temperature conditions, exhaust may be routed to bypass the underbody catalyst to reduce over-heating the catalyst. Also during engine operating conditions such as a DFSO, when there is a possibility of oxygen saturation at the underbody catalyst, exhaust may be routed to bypass the underbody catalyst. Further, a heat exchanger may be coupled to the bypass passage to transfer heat from exhaust flowing through the bypass passage to a coolant circulating through the heat exchanger. The heat recovered at the heat exchanger may be used for providing heat to vehicle components such as a cylinder head, and a passenger cabin.
In this way, by selectively bypassing an exhaust underbody catalyst immediately after an engine cold-start, water condensation and subsequent evaporation at the underbody catalyst may be reduced, thereby decreasing energy dissipation at the catalyst. As a result, delays in catalyst light-off caused by water condensation-evaporation cycles are reduced. In addition, unwanted drops in catalyst temperature from exhaust condensate are reduced. The technical effect of effectively using exhaust heat to expedite water evaporation from the underbody catalyst and increase catalyst temperature is that catalyst light-off is expedited, reducing the use of spark retard for catalyst heating, and increasing fuel economy. The technical effect of bypassing the underbody catalyst during conditions (such as lower than threshold exhaust temperatures and DFSO events) where the catalyst operating temperature may decrease, and/or where oxygen saturation of the catalyst can occur, is that catalyst efficiency may be maintained above a threshold level. By using a heat exchanger in the bypass passage to recover heat from the exhaust, exhaust heat may be effectively used for expedited engine warm-up, and for providing heat to the passenger cabin, thereby reducing the parasitic losses of engine power. Overall, by regulating the flow of exhaust through the exhaust catalyst, and a bypass passage housing a heat exchanger, emissions quality and fuel efficiency may be improved in an engine system.